The present invention relates generally to the system utilized to recirculate electrolyte from the external gas-liquid disengagers to the appropriate electrode frames within an electrochemical cell. More specifically, the present invention relates to an improved recirculation system that connects the appropriate fluid replenisher in the external disengager to each individual electrode frame in a manner which promotes thorough mixing of the recycled and replenished fluids and allows for a controlled concentration gradient through the electrolyte internal circulation loop, as well as minimizing the leakage of electricity in the electrical circuit to ground.
Chlorine and caustic, products of the electrolytic process, are basic chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type electrolytic cell or the flowing mercury cathode-type. Comparatively recent technological advances, such as the development of the dimensionally stable anodes and various coating compositions, have permitted the gap between electrodes to be substantially decreased. This has dramatically increased energy efficiency during the operation of these energy-intensive units.
The development of a hydraulically impermeable membrane has promoted the advent of filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product obviates the need for caustic purification and concentration processing. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, advances continue to be made in the development of monopolar filter press membrane cells.
Replenishing the depleted fluids within the anodes and cathodes has been accomplished in prior art structures simply by having external feed lines carry replenished fluids into the electrodes. These feed lines normally replenish the depleted fluids with fresh fluids by having the external feed lines feed into the top of the appropriate electrode or, in the case of the diaphragm-type cell, into the top of the electrolyte holding vessel.
Prior art structures have also replenished depleted fluids by using internal feed lines. These feed lines replenish the fluids, either deionized water in the case of the cathode or salt brine in the case of the anode, by either utilizing the existing electrode frame side channels to carry the fresh electrolyte towards the bottom of the electrode or feeding the electrolyte into the electrode from the top through short feed lines. An alternative approach is to direct replenished brine into a funnel-type structure connected to a pipe, and then allow the replenished brine to flow to the bottom of the electrolyte holding vessel where the concentrated replenished brine is allowed to mix with existing electrolyte.
All of these methods fail to mix the fresh electrolyte thoroughly with the existing electrolyte in the individual electrodes which are in the electrode holding vessel. These prior art methods fail to provide thorough mixing of the fresh electrolyte with the existing electrolyte before the fresh electrolyte contacts the cell membrane. These methods also fail to provide staged, gradual concentration changes in the electrolyte as it passes through the area or zone of the cell where electrolysis occurs. Lastly, none of the methods provide adequate resistance to the leakage of electrical current to ground. In filter press membrane chloralkali cells this failure to thoroughly mix the electrolyte fluid prior to its entering the individual electrode is even more critical. The nature of the membranes is such that the membranes expand or swell as they absorb the deionized water which is fed into the cathodes from catholyte feed lines. The membranes can also shrink if there is a high concentration of electrolyte, such as salt brine, which tends to dehydrate the membrane. In instances where there is not a thorough mixing of the fresh electrolyte with the depleted electrolyte, the concentration level of electrolyte will vary at different locations throughout the cell. The more concentrated electrolyte tends to dehydrate the membrane in those areas where it is in contact with the membrane. This dehydration tends to shrink the membrane at this point. Such differential swelling and shrinking of the filter press membrane presents operational problems which decrease the operating efficiency of the entire cell.
There is another problem peculiar to filter press membrane chloralkali cells which is created by the addition of fresh brine or other electrolyte chemicals directly into the electrode. The specific problem arises in the anodes of cells employing such a system where the direct addition of these chemicals into the anodes can cause the chemicals to locally attack the membrane. The reaction of the chemicals with the membrane reduces the operating life span of the membrane and generally adversely affects the efficiency of the system.
None of the prior art methods of recirculating and replenishing the cell fluids optimize cell efficiency. Frequently, the methods employed cause excessive dilution of the caustic in the cathodes upon the addition of deionized water. Dilution of the caustic normally occurs where the deionized water is added to the system prior to the withdrawal of the caustic product.
A continuing problem with filter press membrane electrolytic chloralkali cells has been the loss of electrical current due to leakage of electricity to ground. This obviously reduces the overall efficiency of the individual cell.
Additionally, excessive corrosion of the metal feed nozzles within the cell also decreases the energy effectiveness of each unit. More significantly, however, corrosion of the metal parts within the cell may cause fluid leakage, structural damage or plugging of the electrolyte along its path of flow. This corrosion is accelerated by the high electric potential which typically is found in the affected components and can cause extensive outages or repairs.
Attempts to reduce the amount of electrical current leakage to ground led to the use of orifices and other devices in the brine feed line to break the electrically conductive stream of salt brine into droplets prior to its being fed into the cell. These droplets prevent current from traveling up through the electrolyte and out of the cell via the brine feed apparatus. This dropletting of brine has been called the "breaker effect". Such devices have been found effective for smaller electrolyte flows. However, in the large scale commercial production equipment employed today, these devices and others proposed for this purpose are unsatisfactory, frequently proving troublesome and hindering the efficient operation of the cell. Exemplary of the problems encountered in creating this breaker effect is the tendency of orifices and other such devices to become ineffective due to flooding and the increased maintenance that is required because of the larger sized equipment employed. Also, the requirement for large volume capacity equipment in the large sized commercial facilities utilized today has caused the efficient operating potential of such devices to be exceeded.
The foregoing problems are solved in the design of the apparatus comprising the present invention.